Home » Your Car’s Gauges Might Be Lying To You. Here’s How And Why

Your Car’s Gauges Might Be Lying To You. Here’s How And Why

White Lies Gauges Ts
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How much faith do you have in your dashboard? Sure, we’ve all outgrown believing that the biggest number on the speedometer is the top speed of a car, but what about your temperature gauge, your oil pressure gauge, or even the accuracy of your speedometer? The truth is, at least one of your perfectly functional gauges might be lying to you, and gauges lie for a variety of reasons. Let’s hook up a polygraph and see what’s what.

We’re all familiar with those unitless temperature gauges found in most cars over the past few decades, right? At cold start, the needle sits on the cold mark and slowly rises to the middle as you reach normal operating temperature. Well, normal operating temperature isn’t a fixed number at all, but instead a range of temperature. In stop-and-go traffic on a summer day, with low air speed the cooling fans working hard to push or pull air through the radiator, that might be closer to 200 degrees Fahrenheit (94 degrees Celsius), but on the open road at low engine speed, that figure might drop to somewhere near 194 degrees Fahrenheit (90 degrees Celsius) as you have a ton of air passing through the radiator and little power demand from the engine. An engine’s thermostat can do a great job of regulating engine temperature, but a thermostat is only so precise. Some level of minor deviation in operating temperature depending on driving and weather conditions is to be expected.

Vidframe Min Top
Vidframe Min Bottom

Oh, and that’s assuming a vehicle has a conventional engine-driven water pump. For vehicles with electric water pumps, a variety of water pump operation strategies are used, which means that water temperature can do some weird things when you’re just looking at the numbers. For instance, BMW’s N52 inline-six has a targeted temperature for economy, another targeted temperature for normal operation, and two more targeted temperatures for spirited driving. As BMW further explains in a document:

If the vehicle handling causes the engine control unit to detect the economical operating range Economy, the DME regulates to a higher temperature (112 C). In this temperature range, the engine is operated with a relatively [low?] fuel requirement. The friction inside the engine is reduced at higher temperature. The temperature increase thus favours lower fuel consumption in the low load range. In the High and regulation by the map thermostat mode, the driver wants to use optimised power output development of the engine. To achieve this, the temperature in the cylinder head is lowered to 80 °C. This lowering leads to a better cylinder filling, which leads in turn to an increase in engine torque. The engine control unit can now regulate a certain operating range, adapted to the relevant driving situation. This makes it possible to use the cooling system to influence consumption and performance.

Needless to say, BMW didn’t bother with a water temperature gauge for this engine, because normal operating temperature is all over the map. We’re talking about a massive 57.6 degree Fahrenheit swing from the highest targeted temperature to the lowest, after all.

2018 Toyota 4runner 023 6ed6a656b429312b9306b43a2de16d1b7a730e24

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The trouble with natural variances in water temperature being displayed on a gauge is that most humans don’t like to see important needles floating up and down. As soon as that needle goes above the halfway mark, alarm bells start ringing. After all, the average driver might not know how a cooling system works, but they do know what overheating is, so manufacturers often have to build in a calming function. Our own David Tracy explained this as an editor’s note in a previous article on the Mercury Marauder (more on that to come), so I’ll let his words speak volumes.

I worked on the cooling system of the Jeep Wrangler JL, and can tell you that I sat in on a meeting during which we decided how the gauge would behave based on coolant temperature. Yes, “decided,” because the gauge didn’t just go up when things got hot and drop when things got cool, it behaved in a way that reduced the chance of the customer being concerned — for example, it might stay in the “normal range” during a short spikes at, say, a red light at the top of a steep hill that someone towed a trailer up. Was the temperature higher than normal? Sure. But you wouldn’t know it based on the gauge.

The solution, from a manufacturer’s standpoint, is to build a buffer zone into the gauge, where an entire range of normal operating temperatures corresponds to one position on the temperature gauge. It’s become so normalized that owners of all sorts of cars, from Miatas to Camaros to Focus STs to Jaguar XJRs have found buffer zones in their stock temperature gauges. In the beginning, this buffer process used to be carried out by a composite thermistor. According to a patent filed back in 1983, a man named Philip George Camp invented:

A temperature sensor for a motor vehicle comprises a composite of PTC and NTC thermistors (1, 2,3) whose overall temperature characteristic (c) is negative with a plateau (P) centered on the normal working temperature (88°C) to be sensed, so that tolerances in the sensing system and normal fluctuations of the working temperature will tend not to be registered by the vehicle temperature gauge. Thus only abnormal variations in temperature caused by a fault or overload condition will be indicated.

The patent on this invention has since expired, but it’s easy to see why this idea caught on. After all, could you imagine the number of warranty visits caused by a perfectly accurate temperature gauge? In addition, this patent has been referenced in successfully granted patents by automotive giant Siemens, which means it certainly wasn’t a dead end.

Over the past few decades, it’s become the norm for automakers to run instrument clusters entirely electronically, and integrating instrument cluster functions into a module makes building in gauge buffer that much easier. For instance, hash mark positions on the gauge sweep can be assigned hexadecimal values that correspond to a given temperature, with a non-linear sequence of values providing the buffer. Coincidentally, this has led to potential workarounds for buffered temperature gauges, particularly one found by the E46 BMW 3 Series community.

Bmw E46 Interior

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It turns out that on an E46, it’s entirely possible to reprogram the instrument cluster with a different set of hexadecimal values to gain a temperature gauge readout that’s substantially more linear at and above normal street operating temperature. As a forum post on ZHPMafia claims, all that’s needed is software capable of reading from and writing to the instrument cluster module. This should serve the keen driver well, although the sensitivity of the gauge running this set of hexadecimal values might not be to everyone’s taste.

Zhp Coolant Gauge Mod

However, even with buffer zones, modern temperature gauges aren’t entirely useless. They tell us when the engine’s warmed up so it’s okay to rag on it, and they provide (admittedly late) warning of overheating, so if the engine’s shut down right away, damage may be averted. However, what if a gauge doesn’t really have a range? What if it essentially reads “okay” and “not okay”? Well, that’s a dummy gauge. Like buffered gauges, they’re calibrated to make us feel better. Unlike buffered gauges, they aren’t exactly useful.

Mazda Miata Interior

Oil pressure, like coolant temperature, is another gauge readout that’s often manipulated by automakers. See, engines make a different amount of oil pressure at idle versus underway because their oil pumps are mechanically driven — as engine RPMs rise, so does the speed of the oil pump. Manufacturers often specify different normal oil pressures at different engine speeds and oil temperatures, and it’s typically a range rather than absolute values. People who know what the ranges mean typically appreciate accurate data, while people who don’t might not.

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Through the end of the 1994 model year, Mazda equipped the Miata with an accurate oil pressure gauge featuring numeric values and everything. Needless to say, it was awesome. Unfortunately, when the 1995 model year rolled around, the way the gauge worked changed substantially. This new gauge works similarly to the oil pressure gauge in a Mercury Marauder, in that once oil pressure builds to some ludicrously low value, a circuit opens and the gauge needle shoots up. Unfortunately, said ludicrously low value often sits right on the threshold of possible engine damage. We’re talking single-digit pressure here.

Miata Oil Pressure Gauge

Needless to say, many Miata owners prefer the earlier, real oil pressure gauge, and swapping out the later gauge and sender with parts from an earlier model is a fairly common modification discussed reasonably frequently on Miata forums.

Boxster Cluster Nephrite

On a less egregious note, even your speedometer might be lying to you, but only to protect you. I know the one in my Porsche Boxster is. Granted, speedometers aren’t the most accurate way of measuring speed due to fluctuations in tire pressure altering tire diameter and therefore slightly altering detected wheel speed (you can read more about this here), but I’m talking about consistent error. While most speedometers in American-market cars are calibrated to be truthful, that isn’t the case everywhere in the world. According to ECE R-39, part of a major set of automotive safety standards not used by the U.S. or Canada:

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The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.3.5. above, there shall be the following relationship between the speed displayed (V1) and the true speed (V2).

0 ≤ (V1 – V2) ≤ 0.1 V2 + 4 km/h

The part of the equation we’re most interested in is the “+ 4 km/h” part, indicating a four km/h (2.5 miles) positive margin of error baked-in as per regulation. America plays much faster and looser with speedometer calibration, so it’s not uncommon to see high-reading ECE-spec speedometer calibrations in U.S.-market European cars. Unusually, some BMW instrument clusters can be programmed to display true speed, and the previous owner of my 3 Series did just that.

Volkswagen Jetta Dashboard

Some of your gauges might not be telling you the whole truth, but that’s sometimes okay. These little lies keep most drivers comfortable, and can even keep them out of trouble in the case of optimistic speedometers. Besides, if you ever want more accurate data, you can plug in a scan tool to read actual engine coolant temperature, or simply open up your favorite mobile navigation app for GPS-based speed. With easier access to data than ever before, the power of greater accuracy is yours, even if automakers don’t always want to give it to you right away.

(Photo credits: Porsche, Toyota, BMW, Mazda, Volkswagen)

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Bongo Friendee Harvey Park
Bongo Friendee Harvey Park
8 months ago

> In the High and regulation by the map thermostat mode, the driver wants to use optimised power output development of the engine.

Whut

Jim Stock
Jim Stock
8 months ago

I have been running Google Maps with the speed displayed in several different cars and get a consistent 2 mph higher speed on all the car speedometers than shows up on the GPS.

LilRedFinesse
LilRedFinesse
8 months ago
Reply to  Jim Stock

I’ve noticed the same. Also explains why the kind officer always says I was speeding 3 mph less than I was actually speeding.

Phuzz
Phuzz
8 months ago

I already suspected that the temperature gauge in my 2006 Polo was lying, because once it’s warmed up it sit’s rock-solid at 90C. I wonder when the transition was, because my previous (2001) car did have an unbuffered water temp gauge. I don’t think I’ve ever had a car with an oil pressure gauge, just oil temp.

(Oh, and my Polo just passed it’s MoT with zero advisories! Suck on that “VWs are unreliable” people 😉

David Escargot
David Escargot
8 months ago

In Australia speedometers can be out by 10% + 4km/h… that’s up to 14km/h at 100km/h… 8.75mph at 62mph… stupidity really… some people cruise at 10km/h below the limit on their speedo… usually there is an angry trucker sitting right behind them

Old Hippie
Old Hippie
8 months ago

My ’56 VW Transporter had ONE (1) gauge on the dash–not counting the clock, which was wildly fantastic. Everything else was an idiot light. Really confusing was that the Germans consider green light=bad, red light=good. So if that one green light on the dash came on–labeled ENGINE–it didn’t mean “check engine”, it meant “check to see if engine is present”. Likely, it wasn’t. Fuel gauge? Nope. Just another light to tell you to reach down to the right of the driver’s seat and pull out the reserve knob–and start looking for a gas station.

When I put a somewhat hot-rodded 1600 DP in it, I put real gauges on it. All electric, as running an oil line from the engine bay to the dash just wasn’t a good idea. I installed a head temp sensor in each head, an oil temp sensor, and an oil pressure gauge. Now things were terrifying! Oil temp is 275F? Head temps at 300? Oil pressure at idle is 7 psi? 20 full-out? It took a while to train myself that these were good. At least, the ammeter and voltage read where I’d expect them to–I also swapped to 12V at that time.

Now, driving a ’93 Toyota Corolla, the fuel gauge is a constant source of wonder. It works when the rig is off, which I haven’t seen since gauges used wires and rods, and it doesn’t seem to care what level the car is on. It changes really slooooowly. I always wonder what it means to tell me. Fortunately, there’s an amber idiot light that tells me I have about two gallons left, so back to driving it like the old Van with the reserve knob. Or an old motorcycle, for that matter.

Bongo Friendee Harvey Park
Bongo Friendee Harvey Park
8 months ago
Reply to  Old Hippie

Username checks out

Old Hippie
Old Hippie
8 months ago

And proud of it!

Bongo Friendee Harvey Park
Bongo Friendee Harvey Park
8 months ago
Reply to  Old Hippie

Far out, man

Knowonelse
Knowonelse
8 months ago

The speedometer on my ’64 F100 crewcab was badly behaving, reading low mostly, but read some value when at a complete stop. Turns out that the needle itself had bent with age and was rubbing against the glass face! I added some washers to space the speedo away from the glass and now it moves around, but I have no idea of the accuracy.

Nic Periton
Nic Periton
8 months ago

My 1934 Bentley has a lot of gauges, most of them very accurate, I have learnt which to ignore and why. What it does not have is my favorite piece of pedantic gaugery, the temperature gauge in Rolls Royce Phantom III cars is barometrically corrected, as is one of the oil pumps.

TOSSABL
TOSSABL
8 months ago
Reply to  Nic Periton

That’s (nerdly) awesome — especially in that era

Nic Periton
Nic Periton
8 months ago
Reply to  TOSSABL

I suspect that the directors of Rolls Royce were somewhat relieved when a bit of a war broke out, the cars (without bodywork) were sold with a 10 year no quibble guarantee. People quibbled, a lot. I Love these insane things, this recent piece is by someone who shares my enthusiasm.

https://www.classicandsportscar.com/features/rolls-royce-phantom-iii-excellence-all-costs

TOSSABL
TOSSABL
8 months ago
Reply to  Nic Periton

Thanks for that.
I understand the enthusiasm: they were in no way considered disposable when designed or built. I got to examine an earlier one a couple decades back, and the sheer presence of it was almost daunting: when you pull up in one of those, you have Arrived.

Which Bentley do you have? I ask because, in an old Important/Classic cars book I acquired, I actually caught my breath when I opened to the 2-page picture of the Speed6. The lower greenhouse isn’t nearly as stately as the RR, but gives it a subversive—almost ominous air to my eye.
Makes me want to buy lottery tickets.

Nic Periton
Nic Periton
8 months ago
Reply to  TOSSABL

Mine is an 8 Litre with a Thrupp and Mabberly low line four door touring saloon body, the thing has presence! I did not have to buy a lottery ticket as it was passed on to me more than forty years ago, on my 21st birthday. My paternal grandfather bought it new so it has only two names in the logbook, and one surname.

TOSSABL
TOSSABL
8 months ago
Reply to  Nic Periton

Oh, damn, (softly) they are beautiful.
I’ll never have the means —nor am I enough of a badass to sport one—but I truly wish you decades of continued good health to enjoy yours. That’s got to feel special every time you drive it—even after four decades.

Way cool that it’s been in the family since new

Timbuck2
Timbuck2
8 months ago

The only modern vehicle I’ve driven where the oil pressure gauge actually fluctuated with throttle position and temperature was a 2010 Silverado work truck 2500. I don’t know if they made more linear gauges for the work truck variants or what, but it’s the only vehicle I can recall driving that actually showed accurate oil pressure without the buffer. My parents had a Tahoe and the oil pressure gauge never ever varied even the slightest.

John Hower
John Hower
8 months ago
Reply to  Timbuck2

The oil pressure gauges on my old ’07 Silverado and current ’17 Silverado both fluctuated with the throttle. I had a test gauge on the on the ’07 for awhile and it’s readings seemed to match the needle movements on the factory gauges.

Ben
Ben
8 months ago
Reply to  Timbuck2

My Ram’s oil pressure gauge definitely varies with throttle and everything, but I also think it has some smoothing going on. Watching the raw pressure values via the screen they’ll vary by 30 or 40 PSI depending on whether I’m idling or towing up a mountain, but the needle always stays at or above the halfway point on the gauge, even though the alleged range of the gauge means it should be varying more than that.

John J Gerding
John J Gerding
8 months ago

When Ford bought Jaguar and designed the XK8, they put idiot lights on the dash like the cars in the States. Jaguar afficianados had a fit, so they replaced the lights with gauges. But they are, in effect, fake. When I start mine, the oil, temperature and ammeter almost immediately shoot up to the middle of the gauge and stay there unless something bad happens, at which time they shoot to the top of the range. They look nice, but they are totally useless.

Not so, my Durango Hellcat. You can set the dash to show you the exact coolant, oil and transmission temperature. Of course, in the gauge mode, they really don’t move too much, but in the information mode, these numbers should give you the exact reading.

Steve Balistreri
Steve Balistreri
8 months ago

Excellent article and the Speedo gauge offset is right in the money. When we test cars we have to control speed very closely, so we always use a race logic gps speedometer, which consistently reads 3-4 kph lower than the actual speedo.
Something I appreciate about my 94 Bonneville is it has real gauges. The temp, oil pressure, and boost gauge all dance around when you are driving. It’s a nice change from the static modern gauges.
Back in the day when I had a 69 MG midget the speedo would constantly bounce back and forth in a 5 mph arc and I’d just read the average, not that it had the power to go much faster than the speed limit.

Last edited 8 months ago by Steve Balistreri
Col Lingus
Col Lingus
8 months ago

Just recalled something. After 60 years of driving realzed that I owned only 1 car with a factory warranty. And have always had to repair and pay for those repairs myself.
For those reasons I hate gauges that bullshit me.
Always tried to buy the best affordable racing gauges I could. YMMV

R Hum
R Hum
8 months ago

Interesting side note: my 2021 Wrangler has two speedometers. One is a dial and the other is digital. I am not sure how either of them work (are speedometer cables still a thing?). Even thought they are right next to each other, they frequently have different read outs – sometimes by as much as 4 mph. I think one of them is lying to me.

Adam EmmKay8 GTI
Adam EmmKay8 GTI
8 months ago
Reply to  R Hum

I think they usually read transmission’s output shaft velocity to calculate vehicle speeds because all cars I had with open differential would read really high speed when one wheel was spinning on ice and vehicle wasn’t moving.
But In my 2014 Wrangler digital speedometer and gauge are off also. Gauge is about 2MPH faster (almost the 4km/h too) than digital because in USA speedometers cannot indicate to you going slower than actual speed. They have to be calibrated so owners won’t get speeding violations because of bad gauge.
Maybe the digital speedometer reads from ABS sensors (my Jeep does not have GPS)? I’m going to see if digital speedometer reads the same as gauge when I’m on ice next time.
All my domestic cars speedometers always read 2MPH faster than speed from GPS.

PlatinumZJ
PlatinumZJ
8 months ago

Waaaaay back in the day, when my grandma got her first job, her brothers acquired a car for her (supposedly off the black market – this was just after WWII). She was told to keep an eye on the various gauges, and to make sure they stayed pointing up. I don’t know what gauge accuracy would have been like on a vehicle of that era, but I guess that was reasonable advice (although a vertical temperature gauge needle seems like it would be indicating a bit warm).

Captain Zoll
Captain Zoll
8 months ago
Reply to  PlatinumZJ

on older cars i’ve heard it was a common thing to rotate the gauges on wacky angles so that their optimal positions were pointed up.

UnseenCat
UnseenCat
8 months ago
Reply to  Captain Zoll

This was — and still sometimes is — done with individual analog gauges on cars set up for racing. It makes it easier for the driver to scan the gauges quickly; as long as the needle is more-or-less pointing straight up, things are fine. Too far one way or the other, and something needs more attention.

Nowadays with digital telemetry in F1 and other high-tech, high-stakes racing, it’s unnecessary. The crew watching the telemetry will notify the driver on the radio if anything gets out of range.

Adam EmmKay8 GTI
Adam EmmKay8 GTI
8 months ago
Reply to  PlatinumZJ

I think all gauges are set up to have “normal” operating range at 12, 3, or 9 o’clock and point in the same direction when next to each other in a row

Lokki
Lokki
8 months ago

“Meh, good enough.”

Mediocrates
Circa 840 AD

Aaron Crutchfield
Aaron Crutchfield
8 months ago

I suspected something like this was in play with my wife’s 2020 Explorer when we were driving 75-80 mph in 121-degree heat on I-15 in Baker, CA, and the gauge stayed at 4/8 bars even on the long hill climbs. Figured that, or it was the benefit of the car being only one year old.

Hgrunt
Hgrunt
8 months ago

The gauge is most certainly buffered, while mechanically speaking, it’s unlikely it’s secretly running hot–the thermostat will open itself up to regulate the temperature

My GX470 runs at a rock-steady 185F (OBD2 reading) on the freeway regardless of how hot or cold it is outside

Col Lingus
Col Lingus
8 months ago

After several hours of intensive interrogation, it has been revealed that only 1 of my gauges lies. But he’s a pathological one,(and shows remorse) so I know what to expect.
Thankfully my other gauges give a shit and are afraid to lie to my face. None the less I made it clear that I’d be keeping an eye on them just the same.

What gets me though is the little radar signs the cops put up by the side of the roads though that show your speed. If I believe the cops I am going usually way under the limit. But my car says I am already above the limit. Somebody is lying here, and my patience is wearing thin.

And thanks for some good info here. I have always preferred the after market “real” gauges, but the quality is junk now days and just doesn’t feel worth the expense and effort to install.

Last edited 8 months ago by Col Lingus
Adam EmmKay8 GTI
Adam EmmKay8 GTI
8 months ago
Reply to  Col Lingus

I like the “high score” displays (somehow they won’t show high values, and just say “slow down”, lame losers) they install next to 5 lane interstae. Which vehicle in which lane out of 5 is being clocked?

Clark B
Clark B
8 months ago

Ours have a bonus level past “slow down,” which just flashes TOO FAST.

Bhautama
Bhautama
8 months ago

Avg fuel economy readouts are typically programmed to be optimistic as well. Sometimes 5+%. Do the math when you fill up! Meant to account for winter fuel, differing top-off behaviors, etc, to make customers not feel like they’re getting worse than expected.

Adam EmmKay8 GTI
Adam EmmKay8 GTI
8 months ago
Reply to  Bhautama

My fuel tank is 12 gallons and sometimes I can put asnother 1.5 gallons in after fuel pump shuts off. Sometimes I finish fuelling after 1st pump shut off.
THat right there is 10% difference in amount of fuel recorder in my mileage tracking as “full” from user not filling the tank to same amount.
In the last 20 years the car’s display was 2MPG more than what I calculated at most.
In 2022 GTI it is almost spot on, in 2014 Wrangler it is always more by 1 MPG

Col Lingus
Col Lingus
8 months ago

Don’t do that. It wrecks parts of your emissions system, and wastes gas and money.
My wife ran an auto shop for 35 years, and they were constantly seeing this situation.
Google it. Take care.

Last edited 8 months ago by Col Lingus
Adam EmmKay8 GTI
Adam EmmKay8 GTI
8 months ago
Reply to  Col Lingus

I know. I had to replace EVAP solenoids that OBD said were shorted for the fuel tank in my G8 right after that.
With the current fuel prices I can round fuel price to the next full dollar with few drops, and I settle on $x.01 instead of 2nd try to next dollar

Col Lingus
Col Lingus
8 months ago

Good to hear. I usually just round up to next 5 -10 cents.
Good luck.

Oldskool
Oldskool
8 months ago

I’ve done that occasionally, if gas was a really good deal and I’m immediately getting on the road for a long trip.

But any other time, say I just fill it up and go home and park for the rest of the day, the sun beating down on the car can make the gas expand and leak out the overflow. Especially on a Jeep or something with a rear mounted tank. So much for saving money.

So I usually have a really good idea how much it will take to fill it. When I get to like a gallon or two remaining, I’ll pump slowly to cut down on the turbulence that causes a premature shutoff. And stop when it clicks off.

Col Lingus
Col Lingus
8 months ago
Reply to  Bhautama

I try not to get to anywhere with a 5 lane road. But you are right, I hate that crap.

Hgrunt
Hgrunt
8 months ago
Reply to  Bhautama

My GX470’s trip computer is typically 1-1.5 mpg optimistic…which makes the 13.4 MPG in city driving even more painful

Jack Trade
Jack Trade
8 months ago

The freak-you-out-iest version of this has to be the old Porsche oil level gauge – it’s both non-buffered, and renders an accurate reading only when the car is stopped and on a level surface.

Combine this with Porsche’s wonderfully Teutonic (e.g. obtuse) picograms for all the various oil gauges and it’s a complete recipe for a sweep of the eyes across the gauges that results in a total OMG STOP THE CAR RIGHT NOW THE OIL’S IN THE DANGER ZONE! moment.

Cool Dave
Cool Dave
8 months ago

“Sir do you know how fast you were going?”

“I only know what I’m told by my dash officer.”

10001010
10001010
8 months ago
Reply to  Cool Dave

I actually had that happen when I was in college. The speedometer in my $500 Datsun was broken and would just bounce wildly while I was driving so I never knew how fast I was going and eventually got pulled over.
Cop: “Do you know how fast you were going?”
Me: “Well it says 30mph right now and you don’t look like you’re running, so not really.”

AC2DE
AC2DE
8 months ago
Reply to  10001010

My dad had a 1987 Chevy Van that his employees dubbed “Death Van”. Mostly because of the +/- 30 degrees of steering slop. Along with the rust hole through the sheet-metal interior and the non-original back seat mounted with a couple of 2x4s, the speedometer spring was broken. The faster you went, the faster it spun in crazy little circles. If you had perfect pitch, you could gauge your speed from the whine coming out of the differential.

10001010
10001010
8 months ago
Reply to  AC2DE

I tried to judge it off the tachometer in 5th gear. This is before cellphones or GPS though so I’d get a friend to drive next to me with the windows down and shout out the speeds to me so I could make note of which RPMs matched which speeds.

Adam EmmKay8 GTI
Adam EmmKay8 GTI
8 months ago
Reply to  10001010

usually red line in 2nd gear is 62MPH or so, and 3rd gear redlines are usually 90-95MPH

kingRidiculous
kingRidiculous
8 months ago

Tachometers also lie.

Spikedlemon
Spikedlemon
8 months ago
Reply to  kingRidiculous

2006 Yamaha R6 could tell you what you wanted to see.

AC2DE
AC2DE
8 months ago
Reply to  kingRidiculous

Hell, I have a twin-spindle CNC lathe whose spindles can’t agree! If I want both spindles on the same part (to cut it off the bar) I have to command 1500 RPM on the main and 1497 on the sub-spindle. Otherwise one or the other will try to take all the load.

Calibration is hard, even under very controlled circumstances.

H T
H T
8 months ago
Reply to  AC2DE

What kind of lathe? With the subspindle lathes at my company, you turn on the main, then an m-code to speed- or even phase-sync the sub spindle. I’ve never seen individual programming for each spindle when parting off or transferring between spindles.

AC2DE
AC2DE
8 months ago
Reply to  H T

It’s a 2004 Haas TL-15. Sadly, no spindle sync on it, but that just gave me the chance to teach Fusion 360’s post processor to lie. If I want spindle phasing, I issue the appropriate M19 and M119 commands, clamp, then start spindles. Carefully, with fingers crossed.

H T
H T
8 months ago
Reply to  AC2DE

Ah, I get it. Sounds like a good work around.

Drive By Commenter
Drive By Commenter
8 months ago

GM did the same thing with my car. They buried an actual temperature readout into the vehicle functions menu on the instrument panel. It lasted two model years before they canned it for the same reasons listed in the article. It’s genuinely useful since the engine can’t maintain temperature until the coolant hits 160*F.

Nick Ginther
Nick Ginther
8 months ago

What car? My 2000 lesabre will display actual temp and oil pressure.

Some what interestingly, it does not display tire pressure. It just says “normal”

Last edited 8 months ago by Nick Ginther
Drive By Commenter
Drive By Commenter
8 months ago
Reply to  Nick Ginther

2012 Cruze Eco. They didn’t do it for other trims IIRC.

PajeroPilot
PajeroPilot
8 months ago

Ha, snap, I was just about to mention the Cruze! My old car (2013 Holden Cruze SRI-V, that’s a 1.6 petrol turbo – is the Eco the same?) had the numerical temp buried deep in sub-menus – battery voltage as well from memory.

There is a fairly steep, 2-3km climb I have to drive 3 times a week. At the bottom of the hill, the temp would be 104 degrees C, by the time you got to the top it would be 90 (presumably as the cooling system reacted to the load). The normal temperature gauge would be dead vertical the whole time, obviously having a buffer as discussed to prevent owners panicking.

Drive By Commenter
Drive By Commenter
8 months ago
Reply to  PajeroPilot

Not the 1.6, the 1.4 turbo. IIRC those are different engine families and the 1.6 was already pushed to its limits. The 1.4 left lots on the table to run reliably on North American 87 octane gas. Tuning it for 91+ octane delivers a healthy bump in power without affecting reliability. Better fuel economy to boot!

Jack Trade
Jack Trade
8 months ago

Some Fords have this – my 2010 Focus has a secret menu that’ll give you a fair amount of the actuals. It’s a total pain to access (you have do the automotive equivalent of an old PlayStation cheat code sequence) so I rarely use it.

Goof
Goof
8 months ago

I have digital readouts for oil pressure, oil temp, coolant temp, and per tire pressure, plus typical clocks for the tachometer and speedometer (plus digital speed in the tach).

For the temperatures and tire pressure, there’s an update interval. For tire pressure it’s about 10 seconds, for coolant and oil temps it’s about a minute. Oil pressure though? Updates are fast, might be several times a second.

For temperatures I’m sure some hysteresis is baked in so it doesn’t just oscillate needlessly. Seems to be +/- 2C. So those small oscillations get smoothed out.

Tachometer I’ve confirmed with a meter is dead on. Speedometer variance is a function of speed, but it’s fairly accurate. I’ve VMaxed the car (drag limited), and at those speeds it was still within ~10kph of what was GPS-verifiable. Moreover, the max speed on the speedometer is actually legit. The highest number on the speedometer is slightly less than what the car is geared for (6th isn’t an overdrive), but there’s some room past the highest number for the needle to move to where the top of 6th would be. Can’t get there with the car in stock form, but built up motors? I’m sure they get there.

Gee See
Gee See
8 months ago

Also speedometer might not be accurate due to the variance in tire size.. eg BMW e53 X5 can range from 17″ to 20″+

However with everything digital now, it is easier to be accurate / update things like that. The Tesla BLE sensors are slick, it knows every time you change your set of tires (eg summer to winters), it can’t tell what tire size the wheels it is mounted on though, so it can only prompt you. Despite the owner’s shenningans, the company does improve the industry in some unexpected ways.

Last edited 8 months ago by Gee See
Paul B
Paul B
8 months ago
Reply to  Gee See

Most GM’s have a menu through the scan tool that lets you select all the tire sizes that was offered on your vehicle.

Adam EmmKay8 GTI
Adam EmmKay8 GTI
8 months ago
Reply to  Gee See

The wheels sizes were 17″ to 20″. Tire outside diameters were the same on all size’s of wheels. Otherwiese E-class BMW X5 (???) would sit 1.5″ (40mm) lower on 17″ wheels than the one on 20″ wheels.
Only Wrangler, Bronco and some offroad trucks comes from factory with different outside diameter sizes tires. 28″ outside diameter of the tire to 35″ which changes ground clearance and top of the vehicle by 3.5″

Last edited 8 months ago by Adam EmmKay8 GTI
Hgrunt
Hgrunt
8 months ago
Reply to  Gee See

Many car companies use Bosch ABS (I believe Tesla does too) which has high precision wheel speed sensors that can detect very small differences in rotation at each wheel to the point where they can detect whether or not a tire is low on air or any changes at one corner

Some companies ditched dedicated TMPS sensors in some of their cars in favor of using the Bosch system to detect when a tire is low on air because it saves on cost/complexity, and most people probably aren’t looking at the actual PSI anyway

On the Tesla, it’s a combination of that and and clever software. If it detects a sudden change in wheel speeds and behavior from the previous drive, it can prompt you to ask if you changed tires. Another clever feature on Teslas is the tire wear warning, which I imagine works in a similar way

Pat Rich
Pat Rich
8 months ago

I know my gauges are big liars. The Temp buffer thing is a well known “issue” with Land Cruisers. Temp gauge starts moving at 45c and is in the middle from 80c to 105c and it’s in the red at 108c where the AC kicks off and you are in the hurt zone. 3 degrees c…worthless.

Also my speedo is a big fat liar too, showing about 7% more speed than I am actually doing thanks to different gearing and tire size.

I have an old phone running torquePro and hooked up to a reader on my dash all the time so I have a pretty good handle on the actual situation for speed and temps.

OverlandingSprinter
OverlandingSprinter
8 months ago

The dashboard gauges may lie, but do sensors talking to the OBD II report accurate data? I would assume so, but there’s the old saying about making ASSumptions.

Pat Rich
Pat Rich
8 months ago

as accurate as the sensors are I suppose. The variation in readings could be very small or there could be a lot of inaccuracy in the sensor. Also the sensor might not be in a place that tells the whole story. In any case, the sensor output will be way more accurate than the dash.

The Mark
The Mark
8 months ago

OBD will report accurate data. The PCM is sending “60 MPH” to the cluster. The cluster will target the pointer (or digital display) to read 62.

TOSSABL
TOSSABL
8 months ago
Reply to  The Mark

Thank you: I was wondering too.

-I love the breadth and depth of knowledge here

Scoutdude
Scoutdude
8 months ago

As Pat said as accurate as the sensors. In my truck and car I can see The VSS PID as 58 mph when the speedo reads 60, and GPS on my phone also says 58. I can also see the Cylinder head temp rise a couple degrees going up a hill and then drop back down going down the other side, while the gauge stays steady. My wife’s car does have an accurate speedo where the PID matches the display and GPS.

Adam EmmKay8 GTI
Adam EmmKay8 GTI
8 months ago
Reply to  Scoutdude

Right, because your phone’s GPS (which errors vary with how clear or obstructed the sky is) is calibrated instrument like speedometer in your car.
My phones SatNav read my speed at 326MPH when I was driving my Jeep in Colorado

Scoutdude
Scoutdude
8 months ago

I never said that a phone’s GPS is perfectly accurate but I do find that the majority of the time the GPS matches the PID readings across a number of our vehicles. In those same vehicles the speedo reads 2 or 3 mph high whether you are going 35 mph or 75 mph.

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